NF3 treating process

ABSTRACT

A process for treating NF 3  useful as a dry etching gas and cleaning gas in processes for producing LSI, TFT, and solar cell and in an electron photographic processes. The treating process comprises following step: (a) preparing a reactor including agitator blades for agitating gas in the reactor and generating a flow of the gas, and a gas flow guide tube for efficiently circulating and dispersing the gas flow generated by the agitator blades in a space of the reactor; (b) stationarily placing at least one substance selected from the group consisting of a metal and a metal compound within a reactor, the metal being at least one metal selected from the group consisting of Si, B, W, Mo, V, Se, Te and Ge, the metal compound being at least one metal compound selected from the group consisting of solid compounds of Si, B, W, Mo, V, Se, Te and Ge; (c) introducing a gas containing NF 3  into the reactor to react the introduced gas with at least one substance of the metal and the metal compound at a temperature ranging from 400 to 900° C. upon operating the agitator blades of the reactor so as to form a fluoride gas; and (d) capturing the fluoride gas.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a process for treating NF₃ gasthat is useful as a dry etching gas and cleaning gas in processes forproducing LSI, TFT, and solar cell and in an electron photographicprocess.

[0003] 2. Description of the Invention

[0004] NF₃ is a toxic gas having a TLV of 100 ppm that is extremelystable in air and essentially insoluble in water. In the case of usingthis substance, it is necessary at all times to remove residual NF₃present in exhaust gas. Since NF₃ is extremely chemically stable attemperatures near room temperature and is also insoluble in water, itcannot be processed by ordinary gas absorption processes in its originalstate. Consequently, the following process has been proposed in JapanesePatent No. 1538007 (Japanese Patent Provisional Publication No.61-204025),in which NF₃ is reacted with a substance that converts NF₃into a fluoride gas that easily reacts with water and alkaline solution,followed by treating the resulting fluoride gas with a normal gasabsorption process. The Japanese Patent discloses a process wherein NF₃is reacted with Si, B, W, Mo, V, Se, Te, Ge and their non-oxidizingsolid compounds that are used as the converting substance.

[0005] Although the above NF₃ treatment process is effective forconverting NF₃ into an easily treated gas compound, a characteristicreactor taking the NF₃ into consideration was not proposed with respectto the reactor for reacting and treating a large amount of NF₃. Namely,the above Patent only proposes a single flow type of fixed bed reactoras equipment for contacting gaseous NF₃ with a solid compound.

[0006] The fixed bed reactor described in the present specificationrefers to a reactor having a cylindrical outer tube in which a fixedbed, filled with a solid compound such as a metal element that reactswith NF₃ throughout an ordinary cylindrical reactor, is disposed. Thefixed bed is heated as necessary followed by introducing gas from oneend of the cylinder, contacting and reacting the gas with a metalelement and so forth inside the cylindrical tube, and discharging thegas from the other end of the tube. This form of the reactor is thatwhich has been known since long ago. In addition, various types of NF₃detoxification technologies taking into consideration new reactionsystems other than the above reaction system have been disclosed asbeing disclosed in Japanese Patent Publication No. 2-30731 and JapanesePatent provisional Publication No. 7-1555409. In such technologies, thefixed bed reactor of the gas flow type is still used. Thus, the reactorsthat provide an effective setting for an NF₃ detoxification reactionhave not yet discovered.

[0007] Now, treatment of NF₃ gas is accompanied by the generation ofextremely large amounts of heat from the reaction. Namely, its standardformation enthalpy is −127 kJ/mol (−42 kJ per fluorine atom), and in thecase of SiF₄ gas being obtained as the product of the action of metalSi, for example, since the standard formation enthalpy of SiF₄ is −1615kJ/mol (−404 kJ per fluorine atom). The difference between the twoenthalpies are the amount of heat generated accompanying reaction (362kJ per fluorine atom), which demonstrates that NF₃ detoxificationreaction is accompanied by the generation of an extremely large amountof heat. Even though there may be some difference in the amount of heatgenerated in the case that the other substance than Si such as B or W,or if C is selected as a reacting metal element; however, it isintrinsically a reaction that is accompanied by the generation of alarge amount of heat.

[0008] In the case of conducting a gas-solid reaction using a reactor orreaction tube of the type in which gas is allowed to flow over a fixedbed, the reaction initially occurs in the zone on the inlet side of theinitial reaction tube, and as chemical is consumed, the reaction zonegradually moves to the outlet side. Since the flow of gas inside thereactor is so-called piston flow, there are many cases in which thereaction always occurs in a special location inside the reaction tube inthis manner, while other portions of the reaction tube merely fulfillthe role of a gas pathway and are not involved in the reaction itself.Moreover, due to the low rate of heat transfer of the fixed bed, itcannot be said to be suited for efficiently discharging the reactionheat generated locally inside the reactor in this manner outside thesystem.

[0009] For these reasons, when an NF₃ detoxification reaction is carriedout with a fixed bed gas flow system for a reaction that generates alarge amount of heat, the local temperature that results from thereaction ends up becoming extremely high. Consequently, the amount ofNF₃ that be treated per unit time cannot be increased relative to thevolume of the reactor.

[0010] Moreover, there has been proposed a process in which theconcentration of supplied NF₃ is diluted with an inert gas (such as N₂)for the purpose of lowering the temperature of the formed gas. However,this process increases the volumetric flow of all gas resulting in ashortening of retention time, and therefore is not effective as a meansof improving the NF₃ treatment rate per reactor volume. Moreover, evenif a large reactor is attempted to be designed having a larger NF₃treatment rate, there is a limit on the size of the reaction tubediameter for ensuring heat transfer in the radial direction. Ultimately,in order to provide NF₃ treatment volume, a plurality of small diameterreactors must be arranged in parallel, and in any case, fixed bed gasflow systems had the problem of being disadvantageous in terms ofequipment cost.

[0011] In addition, in the case of using Si, for example, in thereaction between NF₃ and Si, a relatively large amount of heat isgenerated on the order of 1,086 kJ/mol. Consequently, this invites alocal temperature rise and overheating in conventional tubularapparatuses of the fixed bed type, thereby placing a limit on the amount(concentration) of NF₃ supplied, and the limit of that suppliedconcentration is 5 vol %. In addition, the actual limit on the tubediameter of a fixed bed system is 150 A (according to JapaneseIndustrial Standard) corresponding to an outer diameter of 165.2 mm.Namely, it was necessary to accompany treatment of NF₃ at 5 NL/min witha diluting gas (N₂) at 100 NL/min. For this reason, fixed bed systemsare not suited for treatment of highly concentrated NF₃ or large amountsof NF₃.

[0012] In addition, in the case of fixed bed systems, treatment capacityhas been observed to decrease when air or oxygen is present.Consequently, there is a need for an NF₃ treatment process that allowstreatment of highly concentrated NF₃, does not result in a decrease intreatment rate even in the presence of, for example oxygen (air) in theNF₃, and is able to ensure a certain degree of treatment volume per unittime.

SUMMARY OF THE INVENTION

[0013] As a result of conducting earnest studies in consideration of theabove-mentioned problems, the inventors of the present invention havefound that highly concentrated and large amounts of NF₃ gas can betreated by creating a setting for gas flow that prevents localoverheating of the fixed portion of a reactor, rapidly transportsgenerated heat to the wall of the reactor with the flow of gas, andprovides as rapid a gas flow as possible along the reactor wall topromote transfer of heat between the gas phase and solid wall in thevicinity of the reactor wall, thereby leading to completion of thepresent invention.

[0014] An aspect of the present invention resides in a process fortreating NF₃, comprising the following step: (a) preparing a firstreactor including agitator blades for agitating gas in the first reactorand generating a flow of the gas, and a gas flow guide tube forefficiently circulating and dispersing the gas flow generated by theagitator blades in a space of the first reactor; (b) stationarilyplacing at least one substance selected from the group consisting of ametal and a metal compound within a first reactor, the metal being atleast one metal selected from the group consisting of Si, B, W, Mo, V,Se, Te and Ge, the metal compound being at least one metal compoundselected from the group consisting of solid compounds of Si, B, W, Mo,V, Se, Te and Ge; (c) introducing a gas containing NF₃ into the firstreactor to react the introduced gas with at least one substance of themetal and the metal compound at a temperature ranging from 400 to 900°C. upon operating the agitator blades of the first reactor so as to forma fluoride gas; and (d) capturing the fluoride gas.

[0015] The above process may further comprises the steps of (e)connecting a second reactor in series with and at a side downstream ofthe first reactor, the second reactor having a fixed bed including atleast one substance of a metal and a metal compound within a firstreactor, the metal being at least one metal selected from the groupconsisting of Si, B, W, Mo, V, Se, Te and Ge, the metal compound beingat least one metal compound selected from the group consisting of solidcompounds of Si, B, W, Mo, V, Se, Te and Ge; and (f) introducing gasdischarged from the first reactor to the second reactor so as to reactthe gas with the at least one substance of the metal and the metalcompound within a temperature ranging from 400 to 900° C.

[0016] Another aspect of the present invention resides in a system fortreating NF₃. The system comprises a reactor which includes an outertube into which a gas containing NF₃ is supplied. Agitator blades aredisposed inside the outer tube for agitating the gas and generating aflow of the gas. A gas flow guide tube is disposed inside the outer tubeto efficiently circulate and disperse the gas flow generated by theagitator blades in a space of the outer tube. Additionally, at least onesubstance selected from the group consisting of a metal and a metalcompound, disposed inside the gas flow guide tube. The metal is at leastone metal selected from the group consisting of Si, B, W, Mo, V, Se, Teand Ge. The metal compound is at least one metal compound selected fromthe group consisting of solid compounds of Si, B, W, Mo, V, Se, Te andGe. Additionally, a heater is disposed outside the outer tube to heatthe space inside the outer tube at a temperature ranging from 400 to900° C.

[0017] According to the NF₃ treatment process and system of the presentinvention, gas containing NF₃ in a large amount and/or at a highconcentration can be adequately removed while performing the treatmentprocess safely without the formation of explosive gas by-products.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 is a schematic longitudinal sectional view of an example ofa horizontal cylindrical reactor used in a NF₃ treating processaccording to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0019] According to the present invention, a process for treating NF₃,comprises the following step: (a) preparing a reactor including agitatorblades for agitating gas in the reactor and generating a flow of thegas, and a gas flow guide tube for efficiently circulating anddispersing the gas flow generated by the agitator blades in a space ofthe reactor; (b) stationarily placing at least one substance selectedfrom the group consisting of a metal and a metal compound within areactor, the metal being at least one metal selected from the groupconsisting of Si, B, W, Mo, V, Se, Te and Ge, the metal compound beingat least one metal compound selected from the group consisting of solidcompounds of Si, B, W, Mo, V, Se, Te and Ge; (c) introducing a gascontaining NF₃ into the reactor to react the introduced gas with atleast one substance of the metal and the metal compound at a temperatureranging from 400 to 900° C. upon operating the agitator blades of thereactor so as to form a fluoride gas; and (d) capturing the fluoridegas.

[0020] The above process is performed by a system including a reactorwhose one example is shown in FIG. 1. The reactor R in FIG. 1 is thehorizontal cylindrical type and comprises an cylindrical outer tube 1having an intake-side end wall 1 a and an exhaust-side end wall 1 b. Atray 2 on which Si granules (treatment agent) are placed is disposedinside the outer tube 1, in which the inside temperature of outer tubeis held at 400-900° C. using an external heater H disposed outside theouter tube 1. Gas containing NF₃ is supplied through a gas intake port 4formed in the intake-side end wall 1 a, and is sufficiently contactedwith the Si by agitating the gas with agitator blades 3 to carry out adecomposition reaction for generating decomposition gas. Under agitationby the agitator blades 3, the decomposition gas and unreacted NF₃ arecirculated in a direction indicated by arrows in FIG. 1 and dispersedthrough a generally cylindrical gas flow guide tube 5. The gas flowguide tube 5 is spacedly disposed between the outer housing 1 and thetray 2. The gas which has been subjected to reaction is discharged froma gas exhaust port 6 formed in the exhaust-side end wall 1 b.

[0021] Since this process allows gas to be more completely mixed thanprocesses of the prior art, it offers the advantage of being able toinhibit local overheating. In this case, it is preferable that the meanflow rate of the circulating gas flow inside the gas flow guide tube 5be 0.5 m/sec or more, and more preferably within the range of 0.5-3.0m/sec. If the mean flow rate is less than 0.5 m/sec, the removal of heatgenerated accompanying the reaction is insufficient resulting in theoccurrence of local overheating. Even if the mean flow rate exceeds 3.0m/sec, the resulting effects of dispersing gas or effects of removingreaction heat are not improved, so that the motive power required foragitation is wasted.

[0022] In addition, although varying according to the shape and size ofthe reactor, it is preferable that the amount of gas introduced into theouter tube 1 is equal to or less than a value (per minute) correspondingto 0.1 times of the internal volume of the outer tube 1 in terms of thevolumetric flow as converted from the standard state. If the volumetricflow or introduced gas amount is greater than this, the heat generatedin the reaction exceeds heat dissipation resulting in overheating of thereactor if NF₃ is highly concentrated at nearly 100%. Conversely, if theNF₃ concentration is low at 50% or lower, the retention time of the gasin the reactor is shortened, resulting in a large amount of unreactedNF₃.

[0023] Moreover, the temperature for carrying out the contactdecomposition reaction of the gas to be introduced into the outer tube 1is preferably within the range of 400-900° C., and optimally within therange of 500-700° C. If the temperature is lower than 400° C., thereaction proceeds slowly and the amount of unreacted NF₃ increases. Inaddition, if the temperature exceeds 900° C., the reaction proceeds toorapidly causing a local reaction which has the risk of damaging themembers of the reactor at that portion.

[0024] The treatment agent used to be reacted with NF₃ in the presentinvention is preferably Si, B, W, Mo, V, Se, Te, Ge or a non-oxide solidcompounds of these metals. Examples of the non-oxide solid compounds ofthese metals are Si₃N₄ and SiC.

[0025] There are no particular restrictions on the material of thereactor R used in the present invention provided it is a metal materialor oxide-based material having corrosion resistance at hightemperatures, in which nickel or nickel alloy is preferable as the metalmaterial. In addition, the shape and dimensions of the reactor R aresuitably selected according to the amount of detoxified substance andthe required detoxification capacity.

[0026] Moreover, in order to carry out NF₃ treatment more efficiently,it is preferable to use a process or treatment (first stage using afirst stage reactor) in which a content of NF₃ of up to several percentis treated with the process of the present invention followed by asecondary treatment (second stage using a second stage reactor) up to 10ppm or less on NF₃ treated in the first stage by a cylindrical reactorof the fixed bed type (piston flow system) filled with the sametreatment agent (such as Si) as that in the reactor R. As a result, alarge amount of highly concentrated NF₃ at a large flow rate can betreated up to the allowed concentration or less.

[0027] SiF₄ is formed when using, for example, Si or Si₃N₄ for thetreatment agent in the first or second stage of treatment. The thusformed SiF₄ can be treated with an ordinary wet scrubber. The wasteliquid that has absorbed SiF₄ in the wet treatment with the wet scrubbercan be treated with a typical process in which the waste liquid is sentto a treatment tank in a later stage in which, for example, a chemicalsuch as calcium hydroxide is added to the waster liquid. At this time,the F and Si are converted to water-insoluble solids such as CaF₂ andSiO₂, respectively, followed by recovery by filtration. In addition,since the gas on which the above treatment is performed is detoxified,it can be purged into the atmosphere.

[0028] As has been described above, according to the process of thepresent invention, by reacting a large amount of NF₃ with a metal and soforth, absorbing the resulting gaseous fluoride (such as SiF₄, BF₃, WF₆,MoF₆ or GeF₄) with a wet scrubber and finally converted to a solid inthe form of, for example, calcium fluoride, the present invention isable to demonstrate detoxifying effects in which gas containing NF₃ isnot discharged into the atmosphere.

[0029] The present invention will be more readily understood withreference to the following Examples in comparison with ComparativeExamples; however, these Examples are intended to illustrate theinvention and are not to be construed to limit the scope of theinvention.

EXAMPLE 1

[0030] A reactor (R) as shown in FIG. 1 was prepared including an outeror reaction tube (1) having a diameter of 400 mm and a length of 1,300mm. The reactor was provided with a tray (2) located inside the outertube as shown in FIG. 1. A gas flow guide tube (5) was disposed betweenthe outer tube and the tray so that the tray was spacedly located insidethe gas flow guide tube as shown in FIG. 1. 55 Kg of metal silicon wereplaced on the tray. The reactor had a reaction portion having a volumeof 200 liters. The reaction tube was heated to 600° C. by an externalheater (H) located outside thereof. NF₃ at 5.0 NL/min was supplied froma gas intake port (4) of the reactor into the outer tube. The retentiontime of the gas in the reactor was 540 seconds. At this time, the gasflow rate of the gas inside the flow guide tube was set to 1.0 m/secusing agitator blades (3) as shown in FIG. 1.

[0031] When a portion of the outlet gas from the gas exhaust port wascaptured and analyzed by a FT-IR (Fourier transform-type infra-redspectroscopy) and a gas chromatography, NF₃, N₂, SiF₄, and N₂O weredetected. The NF₃ concentration at the gas exhaust port was 2 vol %. Theexperiment conditions and analysis results are shown in Table 1 in which“Intake gas concentration” is a concentration (vol %)of the gasintroduced from the gas inlet port of the reactor; “Outlet gasconcentration” is a concentration (vol %) of the gas discharged from thegas exhaust port of the reactor; “Total gas supply volume” is a totalvolume of the gas supplied from the gas inlet port of the reactor, whichare common throughout Examples to Comparative Examples.

EXAMPLE 2

[0032] 55 Kg of metal silicon were placed on the tray in the reactor ofExample 1 followed by heating the reaction tube to 600° C. using theexternal heater and treating the NF₃ contained in the air. NF₃ wassupplied at 8.0 NL/min, N₂ at 6.4 NL/min and O₂ at 1.6 NL/min (total gassupply volume=16.0 NL/min). The gas retention time in the reaction tubewas 170 seconds. At this time, the gas flow rate in the gas flow guidetube was set to 1.0 m/sec using the agitator blades.

[0033] When a portion of the outlet gas from the gas exhaust port wascaptured and analyzed by a FT-IR and a gas chromatography, NF₃, N₂, O₂,SiF₄ and N₂O were detected. The NF₃ concentration was 5 vol %. Theexperiment conditions and analysis results are shown in Table 1.

EXAMPLE 3

[0034] 55 Kg of metal silicon were placed on the tray in the reactor ofExample 1 followed by heating the reaction tube to 600° C. using theexternal heater and treating the NF₃ contained in the air. NF₃ wassupplied at 4.0 NL/min, N₂ at 3.2 NL/min and O₂ at 1.6 NL/min (total gassupply volume=8.0 NL/min). The gas retention time in the reaction tubewas the same as in Example 1. At this time, the gas flow rate in the gasflow guide tube was set to 1.0 m/sec using the agitator blades.

[0035] When a portion of the outlet gas from the gas exhaust port wascaptured and analyzed by a FT-IR and a gas chromatography, NF₃, N₂,O_(2,) SiF₄ and N₂O were detected. The NF₃ concentration was 2 vol %.The experiment conditions and analysis results are shown in Table 1.

EXAMPLE 4

[0036] A nickel vertical cylindrical reactor (latter or second stagereactor) having an inner diameter of 80 mm and a length of 1,050 mm wasconnected to the gas exhaust port of the reactor in Example 2. Sigranules were tightly packed in the latter stage reactor to form a fixedbed in the reactor. The latter stage reactor was heated at 600° C. withthe external heater. Exhaust gas from the reactor in Example 2 wasintroduced into the latter stage reactor having the fixed bed. When aportion of the outlet gas discharged from the latter stage reactor wascaptured and analyzed by a FT-IR and a gas chromatography, NF₃, N₂, O₂,SiF₄ and N₂O were detected. The NF₃ concentration was 10 vol %. Theexperiment conditions and analysis results are shown in Table 1 in which“Intake gas concentrations” was a volume of the gas from the gas exhaustport of the reactor in Example 2. TABLE 1 Total gas Gas Intake gassupply Gas flow concentration volume Retention flow rate Outlet gasconcentration vol % NL/ time guide m/ vol % Reactor NF₃ N₂ O₂ min sec.tube sec NF₃ N₂ O₂ SiF₄ N₂O Ex. 1 Horizontal 100 — —  5 540 Yes 1.0 2 39— 59 — Ex. 2 cylinder  50 40 10 16 170 Yes 1.0 2 54 3 34 8 Ex. 3  50 4010  8 340 Yes 1.0 2 52 3 35 8 Ex. 4 Fixed  2 54  3 17 — — — <10 54 3 358 bed ppm Comp Horizontal 100 — —  5 540 Yes 0.2 10 36 — 54 — Ex. 1cylinder Comp 100 — —  5 540 No 1.0 8 36 — 56 — Ex. 2

COMPARATIVE EXAMPLE 1

[0037] 55 Kg of metal silicon were placed on the tray in the reactor inExample 1, followed by heating the reaction tube to 600° C. using theexternal heater. NF₃ was supplied from the gas intake port at 8.0NL/min. The retention time of gas in the reactor was 540 seconds. Thegas flow rate in the gas flow guide tube was set to 0.2 m/sec using theagitator blades.

[0038] When a portion of the outlet gas was captured and analyzed by aFT-IR and a gas chromatography, the concentration of NF₃ was found to be10 vol %. The experiment conditions and analysis results are shown inTable 1.

COMPARATIVE EXAMPLE 2

[0039] 55 Kg of metal silicon were placed on the tray in the reactorwhich was similar to that of Example 1 with the exception that no gasflow guide tube (5) was provided, followed by heating the reaction tubeto 600° C. using the external heater. NF₃ was supplied from the gasintake port at 5.0 NL/min. The gas retention time in the reactor was 540seconds.

[0040] When a portion of the outlet gas from the gas exhaust port wascaptured and analyzed by a FT-IR and a gas chromatography, theconcentration of NF₃ was found to be 8 vol %. The experiment conditionsand analysis results are shown in Table 1.

COMPARATIVE EXAMPLE 3

[0041] NF₃ was supplied at 5.0 NL/min to the vertical cylinder reactor(having the fixed bed) used as the latter stage reactor in Example 4.The reaction occurred in a confined area near the entrance of thereactor. The reactor walls were damaged after 5 minutes due toaccumulation of reaction heat.

[0042] As apparent from the above, according to the NF₃ treatmentprocess of the present invention, gas containing NF₃ in a large amountand/or at a high concentration can be adequately removed whileperforming the treatment process safely without the formation ofexplosive gas by-products.

What is claimed is:
 1. A process for treating NF₃, comprising thefollowing steps: preparing a first reactor including agitator blades foragitating gas in the first reactor and generating a flow of the gas, anda gas flow guide tube for efficiently circulating and dispersing the gasflow generated by the agitator blades in a space of the first reactor;stationarily placing at least one substance selected from the groupconsisting of a metal and a metal compound within a first reactor, themetal being at least one metal selected from the group consisting of Si,B, W, Mo, V, Se, Te and Ge, the metal compound being at least one metalcompound selected from the group consisting of solid compounds of Si, B,W, Mo, V, Se, Te and Ge; introducing a gas containing NF₃ into the firstreactor to react the introduced gas with at least one substance of themetal and the metal compound at a temperature ranging from 400 to 900°C. upon operating the agitator blades of the first reactor so as to forma fluoride gas; and capturing the fluoride gas.
 2. A process as claimedin claim 1, wherein the preparing step includes preparing the firstreactor including an outer tube in which the gas flow guide tube isgenerally coaxially located so as to form an outer cylindrical spacebetween the outer tube and the gas flow guide tube, and a tray on whichthe at least one substance of the metal and the metal compound isplaced, the tray being located inside the gas flow guide tube, andlocating the agitator blades inside the outer tube and at a sideupstream of the tray.
 3. A process as claimed in claim 1, furthercomprising the step of causing the gas within the gas flow guide tube toflow at a mean flow rate of 0.5 m/sec or more so as to be circulated anddispersed.
 4. A process as claimed in claim 1, further comprising thefollowing steps: connecting a second reactor in series with and at aside downstream of the first reactor, the second reactor having a fixedbed including at least one substance selected from the group consistingof a metal and a metal compound within a first reactor, the metal beingat least one metal selected from the group consisting of Si, B, W, Mo,V, Se, Te and Ge, the metal compound being at least one metal compoundselected from the group consisting of solid compounds of Si, B, W, Mo,V, Se, Te and Ge; and introducing gas discharged from the first reactorto the second reactor so as to react the gas with the at least onesubstance of the metal and the metal compound within a temperatureranging from 400 to 900° C.
 5. A system for treating NF₃, comprising: areactor including an outer tube into which a gas containing NF₃ issupplied, agitator blades disposed inside said outer tube for agitatingthe gas and generating a flow of the gas, a gas flow guide tube disposedinside said outer tube to efficiently circulate or disperse the gas flowgenerated by the agitator blades in a space of the outer tube, at leastone substance selected from the group consisting of a metal and a metalcompound, disposed inside said gas flow guide tube, said metal being atleast one metal selected from the group consisting of Si, B, W, Mo, V,Se, Te and Ge, said metal compound being at least one metal compoundselected from the group consisting of solid compounds of Si, B, W, Mo,V, Se, Te and Ge, and a first heater disposed outside said outer tube toheat the space inside the outer tube at a temperature ranging from 400to 900° C.
 6. A system as claimed in claim 5, wherein the gas flow guidetube of said reactor is generally coaxially located inside said outertube so as to form an outer cylindrical space between the outer tube andthe gas flow guide tube, wherein said reactor includes a tray on whichthe at least one substance of the metal and the metal compound isplaced, the tray being located inside the gas flow guide tube; whereinthe agitator blades of said reactor is located inside the outer tube andat a side upstream of the tray so as to cause the gas to recirculatethrough a space formed inside said gas flow guide tube and through saidouter cylindrical space.
 7. A system as claimed in claim 1, wherein saidagitator blades are arranged to cause the gas within the gas flow guidetube to flow at a mean flow rate of 0.5 m/sec or more.